A standard heater used on, for example, a nozzle of an injection-molding machine comprises a tubular inner wall, a helicoidal heater element, e.g. a resistive heating wire, surrounding the inner tube, a tubular outer wall surrounding the heater element, and a mass of insulating material, e.g. magnesium oxide, filling the annular chamber between the tubes around the heater element. Such a sleeve heater is slipped over the outer cylindrical surface of the normally tubular nozzle and the heater element is connected to a source of electricity. When electricity is passed through the heating element, it heats and this heat is transmitted through the oxide electrical insulation to the inner wall and thence to the outer surface of the object, i.e. the nozzle, the heater is fitted around.
It is, obviously, essential that the sleeve heater fit as snugly as possible around the object being heated for best possible heat conduction between its inner wall and the outer surface of the object. At the same time it must be loose enough that it can be slipped on and of since such a heater is subject to considerable stress and must be removable. Thus there is a tradeoff between the tight fit necessary for good heat transmission and the looseness needed to make removal and replacement possible.